In magneto-optical recording medium frequently embodied in a disk, information in the form of magnetic domains is written on the recording layer having perpendicular anisotropy. This information is read by irradiating the medium with a linearly polarized laser beam and detecting the change of the incident laser beam caused by the interaction of the light with the magnetization. Kerr rotation, coercivity, sensitivity and carrier to noise ratio are critical parameters to be considered for achieving high performance magneto-optical medium.
Currently rare earth-transition metal alloy layers are used as the recording medium. An inherent disadvantage of such layers is that they are highly susceptible to corrosion and oxidation resistance.
When such a layer is deposited directly on the substrate made of organic material such polycarbonate (PC) and polymethylmethacrylate (PMMA), the MO layer does not possess a square Kerr hysteresis loop which is essential for obtaining high performance. For this purpose, the MO recording layer is sandwiched between two layers of ceramics such as, for example, AlN and SiN. These dielectric layers also enhance their environmental stability. However, these dielectric layers are difficult to deposit in a reliable fashion. The reactive sputtering process which is generally used to deposit the dielectric layers may also damage the sensitive substrate and generate mechanical stresses which also may cause the failure of the disk structure. Protective overcoats of active metals such Ti and Cr have been utilized earlier to protect the recording medium eliminating the top overcoat of dielectric (commonly assigned U.S. Pat. No. 4,877,690 to D. Glocker, T. K. Hatwar, S. T. Rao, G. L. McEntyre, and D. G. Stinson). For such a disk the recording and reading was done through the substrate. For front surface recording the metal overcoats can not be used because of their high absorption coefficient.